If you know a little building science, you’ve no doubt seen a lot of problems that occur with air distribution systems. Ducts just don’t get anywhere near the attention they deserve in most homes.
I’ve written about ducts quite a bit here and have shown problems resulting from poor design and installation. We all know how stupid some of those problems are. So today I’m going to talk about a problem that doesn’t get nearly enough attention: duct insulation — even when the design and installation are perfect.
OK, maybe the design isn’t quite perfect. If it were, all the ducts would be in conditioned space. That would be the smart thing to do and would make duct insulation less important. What I’m talking about is a house with ducts in unconditioned space, especially an unconditioned attic.
Our R-value cognitive dissonance
I’m in Georgia (IECC Climate Zone 3), where our current state code requires R-13 in exterior walls, R-18 in attic kneewalls, R-19 in floors, and R-30 in ceilings. If we put ducts in unconditioned space, they have to be insulated to either R-8 (if they’re in an unconditioned attic) or R-6 (in any other type of unconditioned space).
Have you ever considered the temperature differences (delta-Ts) across those different insulated surfaces? Let’s take a look at what they would be on a summer design day here in Atlanta. The outdoor temperature would be 92°F. The indoor temperature would be 75°F. Crawl spaces can vary a lot, depending on how much above-grade foundation wall there is, but 85°F is typical. Attic temperatures are about 120°F. And conditioned air inside a duct is about 58°F. So the delta-Ts would be:
I don’t know about you, but when…
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